I pulled the 6L6s from my Randall and one of the 12AX7s, just to be safe, and installed them in the Peavey, as well as a couple of the original 6L6s that came with it (obviously not the clearly broken one). Turned the amp on, took it off standby and I was greeted by the same crackling and humming that I heard when the amp first arrived
I decided I'd replace one of the original 6L6s with another (seeing as I had 3 to choose from), and turned the amp on, preparing for the same horrific noise and another blown fuse, and it's perfect! Dead silent, no hum, no crackles, and the amp functions exactly as intended 
It seems that 2 of the power tubes were damaged in transit, so no wonder I originally only got about 15 seconds out of it before the fuse blew!
As an added bonus, I've figured out how to get the Sanpera I working and it involves a bit of replugging once the amp is on but that's good enough for me!
This thing is amazing btw, all of this grief has been entirely worth it. I ended up playing for about 6-7 hours yesterday, just discovering what I can (or should I say can't?) do with this amp. Next step is to get it a matched set of 6L6s and a nice new 12AX7 and I imagine it'll be sounding even better than it does now.
Thanks for all the help and advice over the past few days. This forum is full of great guys who know what they're talking about and it's refreshing to find people so willing to try and help!
